Galileo
(scientist) (1564-1642), Italian physicist
and astronomer, who pioneered the
scientific revolution that flowered in the
work of the English physicist Isaac
Newton. His main contributions to
astronomy were the use of the telescope in
observation, and the discovery of lunar
mountains and valleys, the four largest
satellites of Jupiter, the phases of
Venus, and sunspots. In physics, he
discovered the laws governing falling
bodies and projectiles. In the history of
culture, Galileo stands as a symbol of the
battle against authority for freedom of
inquiry.

Galileo,
whose full name was Galileo Galilei, was
born near Pisa, in Tuscany, on February
15, 1564. His father, Vincenzio Galilei,
played an important role in the musical
revolution from medieval polyphony to
harmonic modulation. Just as Vincenzio saw
that rigid theory stifled new forms in
music, so his eldest son came to see both
the currently dominant physics of the
Greek philosopher Aristotle and the Roman
Catholic theology influenced by it as
limiting physical inquiry. Galileo was
taught by monks at Vallombrosa and then
entered the University of Pisa in 1580 or
1581 to study medicine. Although the
syllabus was uncongenial to him, it did
give him a useful introduction to current
versions of Aristotelian
physics.

Aristotelians
made a sharp division between the Earth
and the heavens. In the heavens there
could be no change except the recurring
patterns produced by the circular motions
of the perfectly spherical heavenly
bodies. The sublunar world (the universe
below the Moon) was the region of the four
elements-earth, water, air, and fire-and
subject to its own distinct laws of
natural motion. Fire, for instance, had
lightness, which made it rise vertically,
away from the centre of the Earth. Earthy
objects fell naturally downward towards
the centre of the fixed Earth: the heavier
the object, the faster its fall. "Natural"
motions of the elements took them to their
natural place, where they rested. Rest was
the natural state of an element; it was
motion that needed explaining, since every
motion must have a cause. This
common-sense physics held sway until
Galileo began to undermine it. See
Chemistry: Greek Natural Philosophy;
Philosophy, Greek: Plato and
Aristotle.

Galileo (scientist) (1564-1642), Italian physicist and astronomer,
who pioneered the scientific revolution that flowered in the work of
the English physicist Isaac Newton. His main contributions to
astronomy were the use of the telescope in observation, and the
discovery of lunar mountains and valleys, the four largest satellites
of Jupiter, the phases of Venus, and sunspots. In physics, he
discovered the laws governing falling bodies and projectiles. In the
history of culture, Galileo stands as a symbol of the battle against
authority for freedom of inquiry.

Galileo, whose full name was Galileo Galilei, was born near Pisa,
in Tuscany, on February 15, 1564. His father, Vincenzio Galilei,
played an important role in the musical revolution from medieval
polyphony to harmonic modulation. Just as Vincenzio saw that rigid
theory stifled new forms in music, so his eldest son came to see both
the currently dominant physics of the Greek philosopher Aristotle and
the Roman Catholic theology influenced by it as limiting physical
inquiry. Galileo was taught by monks at Vallombrosa and then entered
the University of Pisa in 1580 or 1581 to study medicine. Although
the syllabus was uncongenial to him, it did give him a useful
introduction to current versions of Aristotelian physics.

Aristotelians made a sharp division between the Earth and the
heavens. In the heavens there could be no change except the recurring
patterns produced by the circular motions of the perfectly spherical
heavenly bodies. The sublunar world (the universe below the Moon) was
the region of the four elements-earth, water, air, and fire-and
subject to its own distinct laws of natural motion. Fire, for
instance, had lightness, which made it rise vertically, away from the
centre of the Earth. Earthy objects fell naturally downward towards
the centre of the fixed Earth: the heavier the object, the faster its
fall. "Natural" motions of the elements took them to their natural
place, where they rested. Rest was the natural state of an element;
it was motion that needed explaining, since every motion must have a
cause. This common-sense physics held sway until Galileo began to
undermine it. See Chemistry: Greek Natural Philosophy; Philosophy,
Greek: Plato and Aristotle.

Galileo's Work in Physics

The key to Galileo's new physics lay in mathematics. Although he
was still registered as a medical student, he increasingly devoted
his time to the extra-curricular study of mathematics, with the
encouragement of the court mathematician Ostilio Ricci. He left the
university without a degree in 1585. For a time he tutored privately
and wrote on hydrostatics, but he did not publish anything. In 1589
he became Professor of Mathematics at the University of Pisa.

The celebrated story of Galileo dropping objects from the Leaning
Tower of Pisa to demonstrate to assembled professors that Aristotle
was fundamentally mistaken about motion comes from his last pupil and
first biographer, Vincenzo Viviani. Though Viviani's account is
sometimes dismissed as legend, it is more probably an exaggerated
version of an actual event. Viviani has Galileo simultaneously
dropping two objects of the same material but different weights to
refute the Aristotelian belief that speed of fall is proportional to
weight. That much Galileo could show even at this early stage of his
career. However, his manuscript works show that he was still unclear
about acceleration in free fall and that he thought more in terms of
the characteristic speed of a body of a given material in a given
medium.

Yet Galileo could already improve on Aristotle. He considered
himself a follower of the ancient Greek scientist Archimedes and
abandoned Aristotelian notions of heaviness and lightness in favour
of the more useful notion of density. He made his first attempts at
producing simple mathematical comparisons of how bodies of varying
densities fall in various media and he was willing to ignore minor
discrepancies, leaving them to be explained by further investigation.
He even toyed with the idea of a body resting on a perfectly smooth
surface being movable by the slightest of forces-a hint of his later
approximation to inertial motion and a measure of how he was
distancing himself from Aristotelian ideas of natural and forced
motions.

Galileo's contract was not renewed in 1592, probably because he
contradicted Aristotelian professors. In the same year he was
appointed to the chair of mathematics at the University of Padua in
the republic of Venice, where he remained until 1610.

At Padua, Galileo invented a calculating "compass" for the
practical solution of mathematical problems. He was much impressed by
the practical knowledge of mechanics displayed by the foremen of the
world-famous shipyard, the Arsenal of Venice. In his own work he
combined an ability to discern simple mathematical patterns
underlying familiar occurrences, such as the free fall of objects to
the ground, with a knack of devising controlled observations in which
the looked-for mathematical relationships presented themselves as
obvious and measurable with precision. His fundamental conviction was
that the universe is an open book but, as he wrote later in The
Assayer, "one cannot understand it unless one first learns to
understand the language and recognize the characters in which it is
written. It is written in mathematical language ."

Projectiles and Pendulums

This conviction led to important discoveries in the first decade
of the 17th century. Galileo not only recognized that the
acceleration of any body in free fall was uniform but he expressed
this in a simple law: the distance travelled in free fall is
proportional to the square of the time elapsed; that is, in 2 seconds
a body will fall 4 times as far as it will in 1 second; in 3 seconds
it will fall 9 times as far; and so on. Alternatively expressed: the
distances moved in successive equal intervals of time are as the odd
numbers: 1, 3, 5 .

This same law led to an understanding of the motion of
projectiles. Galileo could look at the fall of an arrow or cannon
ball and see it as made up of two independent motions: the vertical
component was uniformly accelerated and conformed to his law of
falling bodies; the horizontal motion imparted to the body by the
bowman or gunner was at constant speed. When the horizontal and
vertical components were combined, the resultant path was parabolic.
The practical consequences for efficient gunnery were deduced from
this seemingly abstract geometrical account.

In similar vein, Galileo investigated mechanics and the strength
of materials. In his studies of pendulums he discovered that for a
given pendulum the swing of the bob takes the same time for arcs of
different sizes, though others soon pointed out that this was true
only provided that the swings did not become too large.

One of the greatest contrasts between Galileo's ideas and
Aristotle's is in their underlying models of motion. Galileo
considered that an object moving uniformly on the Earth's surface
without meeting any resistance would continue to do so without
needing to be kept moving by any force, whereas Aristotelians would
look for a force to cause the continuing motion. It is true that the
surface of the Earth is a spherical surface, but it is reasonable to
see Galileo's ideas as approximating to Newton's first law of motion,
according to which a body will continue in its state of rest or
uniform motion in a straight line unless interfered with (see
Mechanics: Newton's Three Laws of Motion). At the least, Galileo made
the advance of not treating rest as a state more natural or
privileged than motion.

Astronomical Research

During most of his Paduan period Galileo showed only occasional
interest in astronomy, although in 1597 he declared in private
correspondence that he preferred the Copernican theory that the Earth
revolves around the Sun to the Aristotelian and Ptolemaic assumption
that the planets, the Moon, and the Sun circle a fixed Earth (see
Ptolemaic System). Only the Copernican model supported Galileo's
ingenious but mistaken theory of the tides: according to this theory
Earth's rotatory motion is alternately added to the orbital motion
and subtracted from it, with the effect that the seas are set
sloshing backwards and forwards. To this simple mechanism, which
provided one tide every 24 hours, Galileo had to add further factors,
such as the orientation and configuration of seabeds and shores, to
make a reasonable approximation to the variety of tidal phenomena
actually observed at different places and seasons.

Discoveries with the Telescope

In 1609 Galileo heard that a telescope had been invented in the
Netherlands. In August of that year he constructed a telescope that
magnified about 10 times and presented it to the doge of Venice. Its
value for naval and maritime operations resulted in the doubling of
his salary and the assurance of lifelong tenure as a professor.

By December 1609 Galileo had built a telescope of 20 times
magnification, with which he discovered mountains and craters on the
Moon. Not only did this contradict the Aristotelian idea that
heavenly bodies must be perfectly spherical; it also indicated that a
heavenly body could be much more like the Earth than had hitherto
been imagined. Galileo also saw that the Milky Way was composed of
stars, and he discovered four satellites circling Jupiter. It was
therefore undeniable that at least some heavenly bodies move round a
centre other than the Earth, a finding that did not prove that
Copernicus had been right, but did fit in well with the Copernican
system of the universe. Galileo published these findings in March
1610 in a book called The Starry Messenger. He astutely used his new
fame to secure an appointment for which he had been angling for some
time, that of court mathematician and philosopher at Florence; he was
thereby freed from teaching duties and had time for research and
writing. By December 1610 he had observed the phases of Venus, which
are a natural consequence of the Copernican system, which has Venus
circling the Sun within Earth's own orbit. The Ptolemaic arrangement,
by contrast, had Venus moving on an epicycle, a circle whose centre
moved around the Earth but was tied to the Earth-Sun line, and it
could not reproduce the phases. Ptolemaic astronomers had to concede
that Venus orbits the Sun rather than the Earth, while still
insisting that the Sun moves around the Earth. Galileo naturally took
the discovery to be a strong confirmation of Copernicanism.

Traditionalist professors of philosophy scorned Galileo's
discoveries because Aristotle had held that only perfectly spherical
bodies could exist in the heavens and that nothing new could ever
appear there. So comets, for instance, had to be assigned to the
world of change below the Moon and treated as meteorological
phenomena. (It is a curiosity that, in a controversy over the comets
of 1618, Galileo, who did as much as anyone to bridge the artificial
gap between Earth and the heavens, was nevertheless willing to treat
the comets as sublunar.)

Galileo also disagreed with professors at Florence and Pisa about
hydrostatics, and he published a book on floating bodies in 1612.
Four printed attacks on this book followed, rejecting Galileo's
physics. Aristotelians took shape to be the key to explaining why
bodies float, whereas Galileo relied on the relative densities of the
floating object and the medium in which it floated. Despite some
embarrassment caused by the fact that he did not understand surface
tension any more than his opponents, Galileo had the better of the
argument, an argument he considered it useless to pursue with
adversaries who were ignorant of elementary mathematics. In 1613 he
published a work on sunspots (see Sun: Sunspots) and predicted
victory for the Copernican theory.

Conflict with the Church

A Pisan professor, in Galileo's absence but in the presence of his
pupil Castelli, told the Medici (the ruling family of Florence as
well as Galileo's employers) that belief in a moving Earth was
contrary to Scripture. Galileo immediately wrote a pamphlet for
private circulation, Letter to Castelli, sketching his views on the
relation of Scripture and science. In December 1614 a Florentine
Dominican denounced "Galileists" from the pulpit, and early in 1615
the Florentine Dominican convent of San Marco sent criticisms of
Galileists to the Inquisition in Rome. Galileo enlarged his Letter to
Castelli into a Letter to the Grand Duchess Cristina on the correct
use of biblical passages in scientific arguments, holding that the
interpretation of the Bible should be adapted to increasing knowledge
and warning against the danger of treating any scientific opinion as
an article of Roman Catholic faith. This remarkable work of amateur
theology was not published in Italy in his lifetime and had little
influence on the course of events.

Early in 1616 Copernican books were subjected to censorship by the
Roman Congregation of the Index of Forbidden Books, after the Jesuit
cardinal Robert Bellarmine had instructed Galileo that he must no
longer hold or defend the opinion that the Earth moves. Following a
long tradition that hypotheses in astronomy were merely instruments
or calculating devices, Cardinal Bellarmine had previously advised
him to treat this subject only hypothetically and for scientific
purposes, without taking Copernican concepts as literally true or
attempting to reconcile them with the Bible. The public ruling of
1616 similarly laid down that Catholics could use Copernicanism as a
calculating device, but could not say that it was the true system of
the universe. Galileo remained silent on the subject for years,
working on a method for determining longitude at sea by using his
predictions of the motions of Jupiter's satellites, resuming his
earlier studies of falling bodies, and skilfully setting forth his
views on scientific reasoning in a book on comets, The Assayer
(1623), which is a classic of polemical writing.

The Trial of Galileo

In 1624 Galileo began a book he wished to call Dialogue on the
Tides, in which he discussed the Ptolemaic and Copernican hypotheses
in relation to the physics of tides. In 1630 the book was licensed
for printing by Roman Catholic censors at Rome, but they altered the
title to Dialogue on the Two Chief World Systems. Because of the
prevalence of plague in central Italy, it was published at Florence
in 1632. Despite the book's having two official licences, Galileo was
summoned to Rome by the Inquisition to stand trial for "grave
suspicion of heresy". Although he had made considerable efforts to
conform to the letter of the ruling of 1616, Galileo had clearly
written a pro-Copernican book. He had occasionally also slipped up by
explicitly treating Copernicanism as "probable", meaning that, though
it was yet unproven, sooner or later it could well be shown to be
true. Such a position was incompatible with the ruling of 1616, as
was pointed out at his trial: Catholics were allowed to use
Copernicanism as a helpful calculating device, provided that they did
not treat it as having any truth in it.

Galileo's legal position was worsened by the presence in his file
of a written but unsigned report that in 1616 he had been personally
ordered not to discuss Copernicanism either orally or in writing.
Cardinal Bellarmine had died, but Galileo produced a certificate
signed by the cardinal, which gave no indication that Galileo had
been subjected to any greater restriction than applied to any Roman
Catholic under the 1616 edict. No signed document contradicting this
was ever found. Galileo was compelled in 1633 to abjure and was
sentenced to life imprisonment (swiftly commuted to permanent house
arrest). The Dialogue was prohibited and the sentence against him was
to be read publicly in every university.

Galileo's Impact on Thought

The condemnation of Galileo did have some effect on universities
and colleges in those countries where the Catholic Church was able to
exercise control of teaching and publication, though the permission
to treat Copernicanism as a useful, though false, calculating device
meant that heliocentric ideas could always be made familiar to
students. The ideas contained in the Dialogue could not be repressed
and Galileo's own scientific reputation remained high, both in Italy
and abroad, especially after the publication of his final and
greatest work.

This was the Discourses Concerning Two New Sciences, published at
Leiden in 1638. It reviews and refines Galileo's earlier studies of
motion and, in general, the principles of mechanics. The book opened
a road that was to lead Newton to the law of universal gravitation,
which linked the planetary laws discovered by Galileo's contemporary
Johannes Kepler with Galileo's mathematical physics. Galileo became
blind before it was published, and he died at Arcetri, near Florence,
on January 8, 1642.

Galileo's most valuable scientific contribution was his part in
transforming physics from a plausible framework erected on casual
observations of complex everyday experiences into a method whereby
selected experiences were so simplified that their underlying
structures or patterns became tractable in geometrical terms and so
susceptible to precise measurement (see Experiment). So, for
instance, the law of falling bodies disregards the resistance of the
medium and concentrates solely on the relationship between distance
fallen and time elapsed in a vacuum. If this simplified law proves to
be only approximate, then the approach is repeated to find what
refinement is needed to account for how an actual body falls-for
example, through air.

Galileo abandoned the key Aristotelian ideas according to which
rest is a natural state and only motion needs explanation, and got so
near to understanding the nature of inertial motion that Newton
credited him with the discovery. More widely influential, however,
were The Starry Messenger and the Dialogue on the Two Chief World
Systems, which opened new vistas in astronomy. He was an outstanding
popularizer of his own work and is recognized as a master of Italian
prose.

Galileo's lifelong struggle to free scientific inquiry from
restriction by philosophical and theological interference is also
remembered as a major contribution to the development of science.
Since the full publication of Galileo's trial documents in the 1870s,
entire responsibility for Galileo's condemnation has customarily been
placed on the officials of the Roman Catholic Church. A fuller
picture would include the role of the professors of philosophy who
first persuaded theologians to link Galileo's science with heresy,
though the responsibility for the ruling of 1616 and for the
condemnation of Galileo must remain with the officials of the Church
and their advisers.

An investigation into the astronomer's condemnation was opened in
1979 by Pope John Paul II. A papal commission, set up in 1982,
produced several scholarly publications related to the trial. In
October 1992 the commission acknowledged the error of the Church's
officials. In a speech accepting the report Pope John Paul, alluding
to Galileo's views on Scripture and science, said that Galileo, "a
sincere believer, showed himself to be more perceptive in this regard
than the theologians who opposed him".